The present invention relates to an arrangement of busbars for conducting direct electric current from the cathode bar ends of one longitudinally disposed electrolytic cell to the anodes of the next cell, in particular on cells for producing aluminum.
The electrolytic production of aluminum from aluminum oxide involves dissolving the latter in a fluoride melt, the greater part of which is made up of cryolite. The cathodically precipitated aluminum collects on the carbon floor of the cell under the fluoride melt, the surface of the liquid aluminum itself forming the cathode. Dipping into the melt from above are anodes which in conventional processes are made of amorphous carbon and are secured to the overhead anode beam. As a result of the electrolytic decomposition of the aluminum oxide, oxygen is produced at the carbon anodes. This oxygen combines with the carbon of the anodes to form CO.sub.2 and CO. The electrolytic process takes place generally at a temperature of approximately 940.degree.-970.degree. C. In the course of the process the electrolyte becomes depleted in aluminum oxide. At a lower concentration of 1-2 wt.% aluminum oxide in the electrolyte the anode effect occurs resulting in a voltage increase from about 4-5 V to 30 V or more. Then at the latest the crust of solidified electrolyte must be broken open and the concentration of aluminum oxide increased by the addition of fresh aluminum oxide (alumina).
The normal mode of operation is such that the cell is usually serviced periodically, even when no anode effect occurs; this means that the crust is broken open and alumina added at regular intervals.
Embedded in the carbon floor of the cell are cathode bars, the ends of which project out of the long sides of the cell. These iron bars collect the electrolyzing current which flows over the busbars situated outside the cell, through the risers, the anode beams and the anode rods to the carbon anodes of the next cell. Energy losses of the order of up to 1 kWh/kg of aluminum produced are caused by the ohmic resistance between the cathode bars and the anodes. Many attempts have therefore been made to optimize the arrangement of the busbars with respect to the ohmic resistance. At the same time, however, one must take into consideration the vertical components of induced magnetic fields which, together with the horizontal current density components, create fields of force in the liquid metal produced in the reduction process.
In an aluminum smelter with end-to-end reduction cells the electric current is passed from cell to cell as follows: The direct electric current leaves the cathode bars which are embedded in the carbon floor of the cell. The ends of the cathode bars are connected via flexible strips to busbars which run parallel to the row of cells. The current is drawn from these busbars, which run along the long sides of the cells, over other flexible strips and risers to both ends of the anode beam of the next cell in the row. Depending on the type of cell, the distribution of current varies from 100-0% to 50-50%, between the nearer and further removed ends of the anode beam, with respect to the general direction of flow along the row of cells. The vertical anode rods which carry the carbon anodes and supply them with electric current are secured by bolts to the anode beam.
This busbar arrangement, which is typical in aluminum smelters is, however, to some extent inconvenient both from the electrical and magnetic standpoint.
The electric current has to be conducted a relatively long distance from the cathode bar ends of one cell to the anodes of the next cell. Viewed in the longitudinal direction of the cell a part of the electric current must be conducted in busbars to the electrically downstream end of the anode beam and then flow back through the beam. Viewed with respect to the vertical direction, the electric current is conducted from the plane of the cathode bars up to the level of the anode beam and then down to the anodes. This forwards and backwards flow of current in two directions means that more metal is required for busbars when the row of cells is built and also that during operation of the cells more energy is consumed due to ohmic resistance in the busbars.
With regard to the resultant magnetic fields the present, conventional method of supplying direct electric current to the cells is not particularly favorable. Three components of metal streaming due to magnetic effects overlap in the cell to produce movements in the liquid metal. These are:
(a) The first component, which is in principle a circular movement along the inner wall of the cell, is especially harmful with respect to the stability of the cell. This first component is caused by the neighboring row of cells which returns the electric current to the rectifier. The direction of rotation depends on whether the neighboring rows of cells lies to the left or the right of the cell in question, with respect to the general direction of current flow.
(b) The second component is due to the fact that in each half of the cell (with respect to the longitudinal direction) there is a circular streaming of the metal, the direction of rotation being different in each half. This type of rotation depends on the distribution of current between the risers.
(c) The third component is made up of rotations in the four cell quadrants, the direction of rotation being the same in diagonally opposite quarters of the cell. These rotations arise from the non-uniform distribution of current in the busbars and anode beam from one end of the cell to the other.
The overlapping of these three streaming components has the result that the rate of flow of metal varies very markedly within the cell. Where all three components act in the same direction the rate of flow of the metal in the cell is high, which causes the carbon lining to be worn away.
It is therefore an object of the invention to achieve an arrangement of busbars for conducting the direct electric current from the cathode bar ends of a longitudinally disposed electrolytic cell to the anodes of the next cell as a result of which less metallic busbar material has to be installed, smaller losses in electrical energy occur and, in addition, the deleterious magnetic effects are diminished.